Epidemiological and family studies have revealed that sibling smoking is more predictive of adolescent tobacco use than parent smoking, and especially in early adolescence may be critically important as a link to peer smoking. Despite this, little is known about the actual interpersonal processes that drive social influence between siblings, especially as they unfold in real time and in everyday settings. Even less is known about ways in which sibling effects interact with other key interpersonal and individual risk factors to propel developmental pathways to tobacco use. The fundamental premise behind this proposal is that studying sibling effects on smoking offers unique opportunities to highlight critical social processes because siblings link broader family risk factors (i.e., parenting) with extrafamilial risk, especially peer influences, during the transition to adolescence. This proposal will use a methodology - Experience Sampling - which is becoming a critically important tool for assessing social interactions, especially as they pertain to health-related behaviors, as they unfold in real time and in everyday settings. Focusing on a cohort of adolescent sibling pairs derived from a community study of tobacco use, Experience Sampling will be used to determine how sibling influences interact with other key inter- and intrapersonal risk factors to promote initial exposure to cigarettes as well as progression of smoking. Five aims will be pursued: (1) To use Experience Sampling to reveal in detail and with ecological validity actual social processes between siblings that serve to facilitate exposure to and use of cigarettes (including exposure to older sibling smoking, direct reinforcing behaviors between siblings); (2) To assess links between sibling influences and key parental risk factors, especially low parental monitoring; (3) To examine if there are particular dispositional characteristics (impulsivity, mood states) that index intrapersonal sensitivity to social influences such as sibling effects; (4) To determine if characteristics of sibling interaction are predictive of later affiliation with peers who use tobacco; and (5) To address theoretical calls in the literature to examine higher-level interactions between these multiple risk factors. Uncovering actual social processes, in real time and in everyday settings, that promote tobacco use will have implications for preventive and interventive strategies aimed at health-related behaviors by providing illuminated and concrete targets for these effects, especially as revealed during adolescence, the key period for initial exposure to tobacco.